Genetically they are the same species, but rainbow trout and steelhead lead very different lives.
These fish represent two separate life strategies of the species Oncorhynchus mykiss: a resident form that stays in freshwater (rainbow trout), and an anadromous form that migrates to the ocean (steelhead). For years, researchers have been unable to identify specific traits that will determine whether a given O. mykiss will exhibit anadromy or residency.
LINK (via: FISHBIO)
